Dopamine (DA) decreases solute and fluid transport in the rat proximal convoluted tubule (PCT). The main objective is to determine the cellular mechanisms for the proximal tubular effect of DA on Na+ transport. It is proposed that DA decreases carrier mediated transport at the brush border membrane (BBM) and active Na+ transport at the basolateral membrane. To determine the activity of DA and DA-1 agonists on Na+/H+ antiport and Na+/Pi symport, the effect on the uptake will be measured in BBM vesicles (BBMV) from rat kidneys. For comparison the effects of DA-2 and alpha-1 adrenergic agonists will also be tested. Specificity wi11 be tested using selective antagonists. DA-1 receptors in PCT are linked to both the phospholipase C (PLC) and adenylate cyclase (AC) systems. It is proposed that the dopaminergic inhibitior of Na+ transport in PCT is linked to DA-1 receptor stimulation of both PLC and AC systems, similar to parathormone effect in proximal tubule. PKC has variable effects on Na+/H+ antiport. Thus, the effect of DA and DA-1 agonists and antagonists on PLC and PKC activities in BBMV will be studied. The linkage between DA-1 inhibition of Na+ transport and activation of PLC will be further tested by examining the effects of DA-1 agonists on Na+ transport after PLC inhibition. The linkage among DA-1 stimulated PLC activity, cAMP, and Na+ will be tested by examining the effect of DA-1 agonists on Na+ transport when cyclic AMP is presert. Exogenous cAMP will be added to BBMV. The role of DA in Na+ transport in PCT will be further studied by examining the 22Na+ uptake in BBMV prepared from rats in which DA synthesis is increased or decreased. Since the effects of DA in the newborn are less pronounced than those in the mature animal despite similar DA-1 receptor density, it is proposed that the difference is due to post-receptor events. Therefore, the effect of DA-1 agonists on Na+ uptake and PLC activity in BBMV obtained from rats of varying ages during maturation will be studied. These studies should lend insight into a cellular mechanism hy which DA- 1 receptor is associated with inhibiton of Na+ transport in PCT. THE RELATIVE CONTRIBUTION OF DIFFERENT SEGMENTS OF NEPHRON TO DA INDUCED NATRIURESIS WILL BE STUDIED BY MICROPUNCTURE TECHNIQUES IN VIVO SINCE LUMINAL MEMBRANES CANNOT BE ADEQUATELY PREPARED FROM DISTAL TUBLAR CELLS.